There are known metal melting furnaces in which fossil fuels are burned with burners using oxygen or oxygen-rich air as a combustion assisting gas, and scraps or ores of iron, copper, aluminum, etc. are melted by the heat of combustion thus generated. Such melting furnaces are described, for example, in Japanese Unexamined PCT Publication No. 501810/1981, and Japanese Unexamined Patent Publication Nos. 215919/1989, 93012/1990, 271804/1993 and 271807/1993.
While these melting furnaces generally are each provided with a melting section where a metallic raw material is melted using oxygen burners and a preheating section where the metallic raw material is preheated, the preheating section is located, in the melting furnaces described in Japanese Unexamined PCT Publication No. 501810/1981 and Japanese Unexamined Patent Publication No. 215919/1989, above the melting section via a closing grid so as to preheat a next charge of metallic raw material on the grid. However, in the metal melting furnace having such iron grid above the melting section, the iron grid is exposed to high temperature, so that it must be cooled with water and the like, causing a great heat loss. These melting furnaces further involve problems of water leakage, troubles in opening and closing the iron grid, etc. due to severe environment to which the melting furnace is exposed.
Meanwhile, in the melting furnace described in Japanese Unexamined Patent Publication No. 271807/1993, which is a so-called reverberatory furnace, a metallic raw material is introduced gravitationally into the melting section while it is preheated by the discharge gas from the melting section when the metallic raw material passes through a slant passage defined in the wall of the furnace. In this case, however, the hot discharge gas tends to flow the upper space of the slant passage serving as the preheating section, so that it is difficult to preheat fully the metallic raw material falling through the lower part of the slant passage, and it is also difficult to control the falling speed of the metallic raw material, because the material is introduced by natural fall.
Generally, in a melting furnace integrated with the preheating section where the metallic raw material is preheated, the rate of introducing the metallic raw material from the preheating section into the melting section significantly influences the heat efficiency. More specifically, the metallic raw material is preferably introduced at the same rate as it is melted in the melting section. If the introducing rate is too high, the large amount of unmelted metal remains as a mixture with the molten metal at the bottom of the melting section, and further there may occur a phenomenon that the molten metal resumes the solid state due to heat loss from the bottom of the furnace. On the other hand, if the introducing rate is too low, it takes much time for introducing the metallic raw material to consume extra energy.
After the metallic raw material is melted in the metal melting furnace, the molten metal in the melting section must be tapped into a ladle and the like. In the case of a relatively small melting furnace, the entire furnace is designed to be tilted to tap the molten metal from a tapping port provided on one side of the melting section. However, in the case of a large melting furnace, since there are problems of securing space for tilting the entire furnace and installation of a large driving mechanism for tilting the furnace, the tapping port is provided at the bottom of the melting section to tap the molten metal through Accordingly, the structure of the melting section is complicated, so that not only the production cost is increased but also maintenance of refractory materials and the like also costs high.
Further, while such metal melting furnaces are generally constructed using large amounts or refractory materials, since the unit requirement of refractory materials which are liable to be damaged influences the melting cost, a water cooling system is incorporated in the case of an electric furnace to cool the furnace using a water-cooling jacket excluding the bottom of the furnace with which the molten metal is brought into contact. The reason why incorporation of such water-cooling system is successful is that, in the structure of the electric furnace, the furnace wall is formed substantially perpendicularly, that the ceiling of the furnace is spaced high away from the furnace bottom, so that there occurs small heat loss even if such water-cooling jacket is used. Meanwhile, in a melting furnace for melting metals using oxygen burners, e.g., the metal melting furnace described in Japanese Unexamined PCT Publication No. 501810/1981, the furnace is partly water-cooled. However, the water-cooled part is only the perpendicular wall of the furnace.
As described above, when water cooling system is incorporated into the metal melting furnace, the part to be water-cooled is limited. Particularly, in the metal melting furnace employing oxygen burners and having a small distance from the surface of the bath of molten metal to the ceiling of the furnace, heat is radiated in large amounts from the molten metal and the burners, and a great heat loss occurs if such water-cooling system is incorporated, impelling use or refractory materials. However, since the melting furnace employing refractory materials is subjected to great thermal shocks in the step of melting the metallic raw material, the frequency that the refractory materials are damaged is increased, increasing the unit requirement of the refractory material and thus influencing the melting cost greatly. Further, it is extremely laborious to form the oxygen burner insertion holes and repair thereof.
Further, in the metal melting furnace employing oxygen burners, fitting positions of the oxygen burners and the directions of injecting flames also significantly influence heat efficiency. More specifically, in melting a metallic raw material using oxygen burners, not only speedy melting of raw material directly by the flames but also preheating of the metallic raw material by the combustion gas are achieved. Accordingly, in order to enhance the heat efficiency, it is essential to carry out preheating of the material fully with the combustion gas and to melt the preheated material speedily by the high-temperature flames, and what is important here is to keep good balance among the melting rate, the preheating rate and the rate of introducing the metallic raw material from the preheating section to the melting section.
For example, although melting performance can be improved by allowing the flames of the oxygen burners to face toward the furnace bottom to some degree, it is almost impossible in an actual melting furnace to attach the oxygen burners to the furnace wall at steep angles approximate to the perpendicular so as to inject flames therefrom toward the bottom of the furnace, and thus the fitting angle of the oxygen burners at the furnace wall has been limited to 10 to 20 degrees with respect to the horizontal, because of the difficulty in forming the burner insertion holes, interference between the attachments of the oxygen burners and the outer wall surface of the furnace. Accordingly, dead zones are liable to be present around the periphery in the furnace, making it difficult to heat the metallic raw material uniformly.
Further, when a metallic raw material is to be melted with the flames of oxygen burners provided above the bath surface, heat can be transferred advantageously to the metallic raw material or the material to be heated in the melting section, since the material to be heated assuming the solid state at the initial stage has a relatively low temperature. However, in the state of liquid or solid-liquid mixture in the middle and later stages of the melting process, not only the temperature of the material to be heated is elevated, but also the heat is transferred only to the surface of the bath and not into the bath, achieving very poor heat transfer. Accordingly, it is a key to improvement in the efficiency of melting metallic raw materials only with oxygen burners to improve heat transfer characteristics at the middle and later stages of the melting process.
Under such circumstances, Japanese Unexamined Patent Publication No. 271804/1993 proposes, as a method of efficiently transferring heat from high-temperature flames formed by combustion at burners to a material to be heated, to allow flames formed by the oxygen burners to impinge upon the material to be heated. However, no matter how the conditions under which the flames are allowed to impinge upon the material to be heated are optimized, the heat transfer area cannot be increased so much, because the bath surface becomes relatively smooth at the middle and later stages of melting, and the gas impinging upon the material to be heated and reflected thereby still has a high temperature, leading to heat loss.
Therefore, it is a first objective of this invention to provide a metal melting furnace which can control the rate of introducing a metallic raw material from the preheating section to the melting section to be within an optimum range, and which can achieve efficient melting of the metallic raw material with oxygen burners only.
It is a second objective of this invention to provide a metal melting furnace which can achieve efficient melting of the metallic raw material with oxygen burners only by preheating the metallic raw material efficiently, and which can facilitate tapping of the molten metal.
It is a third objective of this invention to provide a metal melting furnace which can achieve efficient melting of the metallic raw material with oxygen burners only, and which can reduce unit requirement of refractory materials by introducing a water-cooling system to the portions, to be subjected to high heat load, where oxygen burner insertion holes etc. are defined.
It is a fourth objective of this invention to provide a melting furnace of metals and melting method thereof, which can control the rate of introducing a metallic raw material from the preheating section to the melting section to be within an optimum range, and which can achieve efficient melting of the metallic raw material by well-balanced utilization of the flames formed by combustion at the oxygen burners for melting and preheating the metallic raw material.
It is a fifth objective of this invention to provide a method of melting metals which can transfer efficiently the heat of flames formed by the oxygen burners even at the middle and later stages of melting where the metallic raw material is melted to some degree, and which can achieve efficient melting of the metallic raw material with the oxygen burners only.